Fill valve in locked position won't fill

Yesterday, I replaced the fill valve in my toilet with a Fluidmaster 400LS, and everything in the installation was going fine until I came to the step that reads "Press down on top cap rotating top & arm clockwise to locked position." Until I did this, I could test water flow by turning on the water supply (covering the top of the valve with an upside-down cup so that water wouldn't spray everywhere.) Water is on and the fill valve is filling up the tank nicely. Then when I rotate the top and arm clockwise about 1/4 turn and it locks into position, no water will come out of the valve. When I turn the top and arm a 1/4 turn in the other direction to one of the other locking positions, I get the same result. I've turned the water on and off many times and fiddled with the cap and the locking position, but have had no luck in solving. I can't find any information about this problem online or in another forum post here. Has anyone experienced this before and have any tips on how to solve? Many thanks.

Yesterday, I replaced the fill valve in my toilet with a Fluidmaster 400LS, and everything in the installation was going fine until I came to the step that reads "Press down on top cap rotating top & arm clockwise to locked position." Until I did this, I could test water flow by turning on the water supply (covering the top of the valve with an upside-down cup so that water wouldn't spray everywhere.) Water is on and the fill valve is filling up the tank nicely. Then when I rotate the top and arm clockwise about 1/4 turn and it locks into position, no water will come out of the valve. When I turn the top and arm a 1/4 turn in the other direction to one of the other locking positions, I get the same result. I've turned the water on and off many times and fiddled with the cap and the locking position, but have had no luck in solving. I can't find any information about this problem online or in another forum post here. Has anyone experienced this before and have any tips on how to solve? Many thanks.

I don't know what the pros think of this valve -- most seem to love the basic 400A -- but I'm guessing that they have a higher percentage of successful installs than the DIY-ers in the link above... When I read all the reviews about the tank having no water in it in the morning, I have to wonder whether the valve isn't doing exactly what it's supposed to do, and these people have a slow leak around the flapper that manifests itself in the tank draining overnight. But, hey, no judgments or anything...

Just out of curiosity, it's not clear from your description in what position the float cup is. If it's locked up high, then the valve should not flow water. Once you have rotated the cap into the locked position, is the arm on the top of the valve (and the float cup) dropped into the position that would normally let water flow, or is it locked up by the Leak Sentry device? It's probably a dumb basic question, but sometimes what seems complicated is actually pretty simple.

(Case in point: 20 minutes on phone with cable company the other night trying to get cable box back in business having not used it since night before and now it's showing four dashes across its face. Finally she asks: "Is the Cablecard in the slot?" Well no it isn't. [And let's not discuss why Cablevision still uses this antiquated authentication system; they just still do.] It's gone, and the only people in here that day besides adult family were the cleaning folks. Swell...)

In other words, until the leak-sentry lock that holds the float cup up is released and the float cup slides down the shaft into the fill position, water wouldn't be expected to flow. (I know that you probably know this; just trying to help...)

Beyond that, I'm sure the pros on here have installed thousands of 400As, so maybe they have some insight.

PS I'm curious what regular homeowners see as the value of the Leak Sentry (and its ilk). I totally understand how it might be useful to a landlord whose tenants can be expected to totally ignore a leaky toilet because they are not paying for the water, unless the refill noise keeps them up at night; the value of the valve is that the hassle factor of dealing with an empty tank that the Leak Sentry refuses to refill will likely get the tenants to call the landlord to fix the toilet. (Same thing in a commercial setting, a very large blip in the water bill at a restaurant we owned was traced to a runny employee-only toilet that the manager insisted was working fine.) But in one's own home, the Leak Sentry function wouldn't seem to have much value unless the toilet is in a remote location and isn't used much (we do have a couple of these), or the toilet has primary users who won't react to the leak (like some kids, perhaps). Generally-speaking, don't most homeowners notice a leak pretty quickly without help? All I need to do is hear a toilet refill once on its own to know that it probably needs a new flapper. So, just wondering if I'm missing something.

PPS Not for nothing, but if you can't get this thing to work, it seems to me that it's a heck of a lot easier for a civilian to install a Korky 528 fill valve. As it happens, I just put one in today. Took like 5 minutes, most of which time was spent removing the old valve and removing water from the tank. If you want to compare, here are the installation instructions for the Korky 528: Korky 528 Installation Instructions It's definitely less-involved.